Many existing optical fiber systems are wired with multi-mode optical fiber. Multi-mode fiber has been widely used because of its low price and ease of installation, and because several standards, such as Fiber Data Distribution Interface (FDDI), specify the use of multi-mode fiber. Multi-mode fiber, however, provides a relatively low bandwidth. As higher speed optical systems become available and proliferate, it will become necessary either to replace or to upgrade existing multi-mode fibers, so that high-speed transmission can be supported.
Various techniques have been described for enhancing the capability of existing multi-mode fiber systems. The aim of such techniques is to eliminate the need to replace the existing multi-mode fiber with a more suitable transmission medium, such as single-mode fiber.
One technique for upgrading existing multi-mode fiber systems involved the use of selective optical modes for propagation. In particular, lower-order modes were launched through a single-mode fiber into a multi-mode fiber by direct excitation of the multi-mode fiber. Segments of single-mode fiber were joined to the existing multi-mode fiber either at the transmitting end of the fiber, at its receiving end, or at both ends. The transmitting end single-mode fiber facilitated selective launching of lower-order modes. The receiving end single-mode fiber facilitated filtering of the lower-order modes to eliminate the effects of mode coupling that occurs between the lower-order modes and the higher-order modes as an optical signal propagates in the multi-mode fiber. (As used herein, the term "lower-order modes" refers to modes in which most of the energy is localized around the center of the fiber core, and the term "higher-order modes" refers to modes in which most of the energy is localized outside of the center of the fiber core.) The selective propagation of lower-order modes, however, has not adequately solved the problem of increasing bandwidth while maintaining a low bit-error rate. For example, filtering the lower-order modes only at the receiving end reduces the performance of the system due to the significant loss of energy from the original launch signal. Similarly, filtering the lower-order modes at only the transmitting end reduces the performance of the system due to large modal dispersion. Filtering at both the transmitting and receiving ends renders the system particularly sensitive to mechanical perturbations that increase the bit-error rate.